The present invention relates to measurement of spatial variations in the sensitivity of an optoelectronic imaging device, such as a streak camera, to enable calibration and correction for such spatial variations. The system utilizes uniform illumination of the entrance of the device during measurements. The entrance may be an input aperture, such as a slit, and is referred to as an input slit hereinafter, without limitation to the entrance having another shape. Particularly the invention provides for measurement of the flat-field response representing the variations in sensitivity across an output image of the device and provides a flat-field data set as the system response pixel-by-pixel to a constant in time and spatially uniform illumination source. The illumination source may include light pipes which provide spacial homogenization of the illumination of the input slit during measurements and obtaining the flat-field data set. Geometric distortions can be mapped in accordance with the invention by imposing a spatial modulation using a grid over the output face of the homogenizer. A temporal modulation of the illumination source may also be used.
Streak cameras are useful to make quantitative measurements of transient phenomena lasting typically less than a microsecond. Such streak cameras are commercially available and are described in the literature both as regards their optoelectronics and their sweep deflection circuitry. See, LLE Review, Volume 73, pages 6-73 and Chang, et al., U.S. Pat. No. 5,142,193, issued Aug. 25, 1992; and Kinoshita, U.S. Pat. No. 5,221,836, issued Jun. 22, 1993. Images from streak cameras may be obtained using CCD cameras as their recording medium thereby enabling quantitative measurements of the phenomena to be made with higher precision and higher signal-to-noise ratios than previously was the case with film as the recording medium. In accordance with invention, the image obtained by a CCD camera and particularly a cooled, scientific grade CCD, provides for accurate photometric calibration of the streak camera and when calibrated, accurate measurements of the phenomena can be made with the streak camera.
Various proposals have been made for calibration of streak cameras. Most require the use of calibrating illumination of high intensity and submicrosecond duration. See an article entitled xe2x80x9cFlat-field Response and Geometric Distortion Measurements of Optical Streak Cameras,xe2x80x9d by D. S. Montgomery et al. which appeared in SPIE, Vol. 832, High Speed Photography, Videography and Photonics V (1987, 283-288). Other proposal for calibration may be found in the following U.S. Patents, U.S. Pat. No. 4,628,352 issued to Boue, Dec. 9, 1986; Tsuchiya, et al., U.S. Pat. No. 5,043,568, Aug. 27, 1991; Koishi, et al., U.S. Pat. No. 4,945,224, Jul. 31, 1990; Oba, U.S. Pat. No. 4,714,825, Dec. 22, 1987; Schiller, et al., U.S. Pat. No. 4,435,727, Mar. 6, 1984; LeBars, et al., U.S. Pat. No. 5,118,943, Jun. 2, 1992; Arseneau, U.S. Pat. No. 4,323,977, Apr. 6, 1982; Stoub, et al., U.S. Pat. No. 4,298,944, Nov. 3, 1981; Malueg, U.S. Pat. No. 3,949,162, Apr. 6, 1976; Prager, et al. U.S. Pat. No. 5,726,915, Mar. 10, 1998; Therrien, U.S. Pat. No. 4,523,231, Jun. 11, 1985; Knoll, et al., U.S. Pat. No. 4,386,404, May 31, 1983; and Knoll, et al., U.S. Pat. No. 4,212,061, Jul. 8, 1980. The latter two patents disclose calibration of scintillation cameras which are examples of other optoelectronic imaging devices than streak cameras.
Proper calibration requires one to account for any localized differences in the recorded signal due to distortions, aberrations and defects in any of the streak camera components. To this end, one must measure very accurately the spatial variations in sensitivity across the output image, i.e., the flat-field response, and then perform a flat-field correction to the signal data set. The flat-field data set is the system response, pixel-by-pixel, to a constant in time and spatially uniform illumination source. The correction consists of dividing the signal data set by a normalized flat-field data set. Localized or small scale length variations in the system response could be due to photocathode or phosphor screen non-uniformities, differences in individual CCD pixel sensitivity, defects in the fiber optic window, etc. Long-scale length variations could be due to vignetting in the input optics or the electron optics, or to differences in the photocathode quantum efficiency along the input slit.
Calibrations must also include mapping any geometric distortions in the output image and correcting for them. Geometric distortions originating in the streak tube may be caused by the use of curved input or output surfaces, electron-optical spherical aberration or mechanical misalignment of the electrodes. Fiber optic components with twists or shears may also introduce geometric distortions. The calibrations discussed herein do not extend to establishing iso-temporal contours in the output image, or to corrections for nonlinear streak speeds. The techniques for calibrating streak speeds are well known.
Streak camera records in general have limited SNR due to the small number of photoelectrons per time and spatial resolution element that make up the signal. Excess current will introduce nonlinearities in the photoelectron beam that cannot be corrected. The photocurrent is restricted by space charge effects while the electrons are in transit from the photocathode to the screen and by charge depletion in the photocathode. In order to achieve the best SNR data, streak tubes are always operated close to their peak current handling capability. However, the system""s flat-field response must be obtained with a SNR much greater than that of the signal data set so that the SNR of the corrected data set will not be significantly degraded. This is most important in those regions of the image where the sensitivity is poor to begin with. Since the SNR of a single flat-field data set can be no better than for a single signal data set, if they both are acquired with the same streak duration, multiple flat-field images must be collected and averaged to achieve the requisite SNR. This is regardless of the immense difficulty in producing a high brightness, constant amplitude light source with duration ranging from a few nanonseconds (ns) to a microsecond. Limited SNR also affects the precision with which the geometric distortions can be mapped and thereafter corrected.
In a streak camera that incorporates a fiber optically coupled, back-illuminated CCD camera (and no image intensifier), a streak tube photoelectron generates typically 25 CCD electrons. If the CCD pixel full well is 250,000 to 300,000 electrons, (typical of a 24 xcexcm square pixel), the single pixel SNR is limited to about 100 by Poisson statistics. The regions of the image where the sensitivity is poor will have a lower SNR.
To record a flat-field image that has a SNR of 100 on a Megapixel CCD array requires that 1.6 nC of charge be extracted from the photocathode, (104 photoelectrons per pixel times 106 pixels). The time duration for extracting this amount of charge is limited by the current handling capability of the streak tube, the charge stored in the capacitor formed by the photocathode and accelerator electrodes and the charge replenishment rate from the power supply. Preferably the electrode potentials should not change by more than 1% during the flat-field image acquisition time period to avoid affecting the focusing of the electron optics. The peak photocurrent delivered to the screen is typically less than 1.6 mA for commercially available streak tubes. This is calculated as 1% of the Child-Langmuir space charge limited current density at the photocathode times the usable photocathode area times the fraction of photoelectrons emitted from the photocathode that contribute to the signal at the screen. A simple division of the charge required by the peak current gives a minimum flat-field acquisition times of 1 xcexcs. The total charge stored in the photocathode is typically less than 16 nC. Therefore, one would have to rely on charge replenishment from the power supply in order not to defocus the electron optics due to charge depletion at the photocathode, if extracted at the peak current rate. Typical streak tube power supplies draw about 100 xcexcA in the resistor divider circuits establishing the cathode and other electrode potentials. Therefore, only about 1 xcexcA is available for the photocurrent under continuous operation without defocusing the tube. This limits the single image flat-field acquisition time to greater than 1 ms. Brute force techniques such as adding external capacitors to the photocathode accelerator electrode gap, or utilizing higher current power supplies will aid in decreasing the flat-field acquisition time, but only to the extent that the charge replenishment rate to the region from which the charge is extracted is not limited by the photocathode resistivity.
It is the general object of the invention to provide a system for measuring in a single image the spatial variations in the sensitivity of the recorded output (the image) of a optoelectronic device, and especially for providing a signal for calibrating an optoelectronic imaging device such as a streak camera with an SNR greater than may be achieved in a single submicrosecond streak record.
Another object of the invention is to provide an improved system in which spatial sensitivity variations are acquired during a 1 ms to 10 second duration linear streak ramp in a streak camera thereby facilitating flat-field measurements and to measure the linearity of such ramps thereby insuring linearity thereof. A 1 second ramp is presently preferred.
It is a further object of the invention to provide improved optics especially adapted to be used to produce a spatially-uniform illumination source for flat-field measurements in streak cameras and otherwise.
It is a still further object of the invention to provide in a system for measuring spatial variations in the sensitivity of a streak camera or the recorded output of other optoelectronic imaging devices, and can have the means to measure geometric distortion in the image (the streak camera record).
A still further object of the present invention is to provide a system for measuring spatial variations in the response of an optoelectronic device, such as a streak camera, and acquiring a flat-field image for measuring the variation and obtaining a calibrating signal which bypasses the current and stored charged limitations discussed above by utilizing a slow sweep or streak ramp during acquisition of the flat-field image.
The present invention is based upon the discovery that spatial variations in sensitivity as represented by the flat-field image are independent of the rate that the image is acquired. In accordance with the invention, a slow scan (sweep) or streak ramp is used which may nominally be one second in duration while the device, particularly the photocathode of the streak camera being calibrated is illuminated with a constant uniform source field. The ramps may vary from one millisecond to ten seconds in duration. The maximum streak ramp duration is limited by the collection of dark current from either the streak tube photocathode or the camera (the CCD). High SNR is thereby obtained. SNR is maximized by choosing a ramp duration and light level so that the peak value of the recorded flat-field image approaches the CCD pixel full well. Better SNR may be obtained by acquiring and averaging multiple images.
The light source for flat-fielding streak cameras must be stable, emitting a constant fluence during the streak duration. Ideally, it must have the same spectrum as used for collecting the signal data set. LED""s or laser diodes can be used for the source, as can a simple DC white light source with an appropriate color filter. The slow ramps permit the use of mechanical shutters to gate and/or modulate the light source. The high degree of spatial uniformity required for the flat-field measurements is achieved by using rectangular glass rods to spatially mix or homogenize the input light source. These glass rods have a reflective coating on their sides, and function as four mirror kaleidoscopes. The homogenizers may be either solid or hollow core light pipes. The light is fed into the homogenizer by an array of optical fibers, and typically makes several bounces before exiting as a spatially uniform source field. The rectangular cross section is matched to the photocathode area, typically 1-3 cm by less than 1 mm. Homogenizers may be stacked lengthwise with diffractive optic elements or diffusers in between them for additional smoothing of the spatial profile, especially when coherent light sources are used. The flat sides of the homogenizer ensure that they do not have any optical power that can produce virtual sources inside the homogenizer. However, the end facet of the homogenizer may be shaped to form a non-planar object plane for the streak camera.
Mapping the geometric distortions of the streak camera can be accomplished with the slow streak ramp technique by imposing a spatial and temporal modulation on the input source field. This produces a two dimensional array of bright areas in the output image that when corrected for the geometric distortion should form a linear grid pattern. It is not required that the spatial or the temporal modulations be periodic, although it does simplify the correction algorithm. A known spatial modulation is imposed on the input source field by placing a grid over the output fact of a homogenizer. The temporal modulation of the light source is produced electronically or by a mechanical chopper.